XV
If you have advanced a paradoxical proposition and find a difficulty in proving it, you may submit for your opponent's acceptance or rejection some true proposition, the truth of which, however, is not quite palpable, as though you wished to draw your proof from it. Should he reject it because he suspects a trick, you can obtain your triumph by showing how absurd he is; should he accept it> you have got reason on your side for the moment, and must now look about you; or else you can employ the previous trick as well, and maintain that your paradox is proved by the proposition which he has accepted. For this an extreme degree of impudence is required; but experience shows cases of it, and there are people who practise it by instinct.
XVI
Another trick is to use arguments ad hominem, or ex concessis14 When your opponent makes a proposition, you must try to see whether it is not in some way if needs be, only apparently inconsistent with some other proposition which he has made or admitted, or with the principles of a school or sect which he has commended and approved, or with the actions of those who support the sect, or else of those who give it only an apparent and spurious support, or with his own actions or want of action. For example, should he defend suicide, you may at once exclaim, "Why don't you hang yourself?" Should he maintain that Berlin is an unpleasant place to live in, you may say, "Why don't you leave by the first train?" Some such claptrap is always possible.
XVII
If your opponent presses you with a counter-proof, you will often be able to save yourself by advancing some subtle distinction, which, it is true, had not previously occurred to you; that is, if the matter admits of a double application, or of being taken in any ambiguous sense.
XVIII
If you observe that your opponent has taken up a line of argument which will end in your defeat, you must not allow him to carry it to its conclusion, but interrupt the course of the dispute in time, or break it off altogether, or lead him away from the subject, and bring him to others. In short, you must effect the trick which will be noticed later on, the mutatio controversiae. (See § xxix.)
XIX
Should your opponent expressly challenge you to produce any objection to some definite point in his argument, and you have nothing much to say, you must try to give the matter a general turn, and then talk against that. If you are called upon to say why a particular physical hypothesis cannot be accepted, you may speak of the fallibility of human knowledge, and give various illustrations of it.
XX
When you have elicited all your premisses, and your opponent has admitted them, you must refrain from asking him for the conclusion, but draw it at once for yourself; nay, even though one or other of the premisses should be lacking, you may take it as though it too had been admitted, and draw the conclusion. This trick is an application of the fallacy non causae ut causae